All pieces of vibratory equipment, such as conveyors and feeders, have associated "static" and "dynamic reaction" forces at the structural support points. The "static reaction" forces are simply the forces of the weight of the equipment on the supporting structure. These static forces do not vary significantly over time. The "dynamic reaction" forces are those forces on the supporting structure that are produced when the equipment is operating. The dynamic forces vary in time, are dependent on the operating frequency of the equipment (which is determined by the stiffness of the springs used in the mounting of the equipment and the amplitude of the movement of the equipment), are fully reversing (tension/compression forces), and typically occur at a relatively high frequency of about 5-30 Hz.
For many end-users of vibratory equipment, one of the primary concerns when designing a new manufacturing plant, and associated buildings, are the dynamic reaction forces produced by the vibratory equipment. The supporting structures have to handle both the magnitude as well as the frequency of the dynamic forces. Because buildings have their own natural frequencies, the impact of the operating frequency of the equipment, in particular, must be taken into consideration. If one of the natural frequency modes of vibration of the building is in close proximity to the operating speed of the equipment, there can be some significant "dynamic amplification." This can cause the vibration levels to become quite high, thereby creating safety hazard and discomfort for persons working within the building.
It is known in the art that dynamic vibration absorbers can be used to essentially remove one component of the dynamic force to alter the net direction of the force on the equipment. For example, in U.S. Pat. No. 3,834,523, issued to Evans, a resiliently supported vibratory conveyor is guided along an inclined path of vibration by dynamic vibration absorbers the springs of which are firmly attached to the conveyor structure. The absorbers serve as the equivalent of rigid links from a fixed support to confine the vibratory motion of the conveyor to the desired inclined path without transmitting substantial vibratory force to the support structure. However, systems of this type do not attempt to reduce the dynamic reaction forces to essentially zero. They simply alter the direction of the forces, and hence, the detrimental effect of the forces must still be addressed with regard to the building structure.
An early excellent attempt at addressing the problems of dynamic forces acting upon the support structure for a vibrating apparatus is found in U.S. Pat. No. 3,668,939, issued to Schrader. This patent describes a two-degree of freedom dynamic absorber used to generate--at the operating frequency--a counterforce that opposing the vibration producing forces applied to a conveyor base in both the vertical and horizontal directions, regardless of the phasing or timing between the forces. The absorbers include an upstanding flange mounted on a bracket attached to the base of the conveyor. Extending from either side of the flange is a pair of springs, with a weight mounted at the end of each spring. The weights and springs, in combination, form a vibratory system constituting a dynamic absorber the natural frequency of which in any direction parallel to the flange is closely equal to the frequency of the vibratory forces applied to the bracket, and hence the conveyor, which forces are to be counterbalanced. While the objective of the absorbers of the '939 patent to effectively counterbalance the vibratory forces applied to the base which vary in both amplitude and direction according to the load on the conveyor was addressed, the absorbers are somewhat limited with respect to the frequency range over which they can be used. When the spring and weight absorbers are constructed to operate at low frequencies, they tend to occupy considerable space, making them impractical. The smaller, more space saving absorbers function well at low frequencies, but are very sensitive to changes in the operating speed of the system, and can cause severe structural damage if operated outside the narrow operating band.
Thus, a paramount objective is to devise a broad band absorber that is not extremely sensitive to changes in the operating speed of the system, requires relatively little space for installation, and is relatively inexpensive to install.